Before spider mites mate, one of them gets their skin removed

It’s no secret that breeding makes for fierce competition in nature—even for creatures that clock in at about 1/25th of an inch long. A study published July 7 in the journal iScience finds that male spider mites will guard premature females that are getting ready to molt and mature. Then, at maturation, they’ll actively strip off their skin.

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Competition for first mating is especially intense. The first copulation partner of a female spider mite is the one who will sire the rest of their offspring and also determines the sex of such offspring. Male spider mites sire the female baby spiders, while young males only arise from unfertilized eggs. This intense competition to be top spider mite male is likely what leads some of them to watch over the females for several hours before she molts into her adult stage. 

Roughly one or two hours before molting, female spider mites have a silvery appearance due to the air filling the gap between their exuvia (old skin) and their new skin. During this phase, guarding males will change their behavior. They will sometimes drum on the females with their forelegs, attempting to bulge and crack the exuvia to stimulate an early molt. 

“Such undressing behavior by the male is adaptive—that is, it increases their reproductive success—because it would be an enormous cost to the guarding male if a rival would take away the female and inseminate her instead of the male that invested time and energy in guarding her,” study co-author and University of Vienna biologist and behavioral ecologist Peter Schausberger said in a statement. “The guards would have invested hours in guarding a potential future mate without any reward.”

Once the exuvia is cracked, the guarding male becomes very active and pulls on the hind part of the female spider’s old skin with his pedipalps until it is completely removed and the female’s genital opening is exposed. The female’s geneital opening is located on the underside of the tip of her abdomen. When it is out in the open, the male can slip beneath the female and insert his reproductive organ called the aedeagus.

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“Females that are undressed by a male first get rid of the hind part of the old skin because of male pulling, whereas females that molt without the help of a male first pull out from the front part of the old skin,” said Schausberger.

The team on this study observed and recorded numerous male-male and male-female interactions in spider mites when they noticed this behavior. They say it is another example of the unique behaviors driven by sexual selection and a reminder that even small arachnids have highly sophisticated rituals.

In future research, the team hopes to examine this behavior in greater detail to see whether fighting males differ from the ones that sneak up on mates. They also hope to find out when males who remove females’ skin have to fight off rivals and if the act is a signal’s the male’s quality as a mate to the female.